Transthyretin (TTR, for “transports thyroxine and retinol” and originally named “prealbumin”) is a serum and cerebrospinal fluid carrier of the thyroid hormone thyroxine (T4) and retinol binding protein bound to retinol. The liver secretes transthyretin into the blood, while the choroid plexus secretes TTR into the cerebrospinal fluid. It functions with two other thyroid hormone-binding proteins in the serum, thyroxine-binding globin (TBG) and albumin.
TTR is a 55 kDa homotetramer with a dimer of dimers quaternary structure that is synthesized in the liver, choroid plexus and retinal pigment epithelium for secretion into the bloodstream, cerebrospinal fluid and the eye, respectively. TTR misfolding and aggregation is known to be associated with the amyloid diseases (Zeldenrust and Benson. In Ramirez-Alvarado M, Kelly J W, Dobson C. Protein misfolding diseases: current and emerging principles and therapies. New York: Wiley) senile systemic amyloidosis (SSA; Westermark et al. Proc. Natl. Acad. Sci. U.S.A. 87: 2843-5), familial amyloid polyneuropathy (FAP; Andrade C. Brain 75: 408-27; Coelho T. Curr. Opin. Neurol. 9: 355-9), and familial amyloid cardiomyopathy (FAC; Jacobson et al. N. Engl. J. Med. 336: 466-73).
TTR tetramer dissociation is known to be rate-limiting for amyloid fibril formation (Colon and Kelly. Biochemistry 31: 8654-60; Lai et al. Biochemistry 35: 6470-82; Hammarström et al. Science 299: 713-6). However, the monomer also must partially denature in order for TTR to be mis-assembly competent, leading to a variety of aggregate structures including amyloid fibrils (Jiang et al. Biochemistry 40: 11442-52). While wild type TTR can dissociate, misfold and aggregate leading to SSA, point mutations within TTR are known to destabilize the tetramer composed of mutant and wild type TTR subunits facilitating more facile dissociation and/or misfolding and amyloidogenesis (Sekijima et al. Cell 121 (1): 73-85). A replacement of valine by methionine at position 30 (TTR V30M) is the mutation most commonly associated with FAP (Saraiva M J. Hum. Mutat. 5: 191-6). A position 122 replacement of valine by isoleucine (TTR V122I) is carried by 3.9% of the African-American population, and is the most common cause of FAC (Jacobson et al. N. Engl. J. Med. 336: 466-73). SSA is estimated to affect over 25% of the population over age 80 (Westermark et al. Proc. Natl. Acad. Sci. U.S.A. 87: 2843-5). Severity of disease varies greatly by mutation, with some mutations causing disease in the first or second decade of life, and others being more benign. Deposition of TTR amyloid is generally observed extracellularly, although TTR deposits are also clearly observed within the cardiomyocytes of the heart.
Both normal-sequence TTR and variant-sequence TTR cause amyloidosis. Normal-sequence TTR causes cardiac amyloidosis in people who are elderly and is termed senile systemic amyloidosis (SSA) (also called senile cardiac amyloidosis (SCA)). SSA often is accompanied by microscopic deposits in many other organs. TTR mutations accelerate the process of TTR amyloid formation and are the most important risk factor for the development of clinically significant TTR amyloidosis (also called ATTR (amyloidosis-transthyretin type)). More than 85 amyloidogenic TTR variants are known to cause systemic familial amyloidosis. The liver is the major site of TTR expression. Other significant sites of expression include the choroid plexus, retina and pancreas. TTR amyloidosis manifests in various forms. When the peripheral nervous system is affected more prominently, the disease is termed familial amyloidotic polyneuropathy (FAP). When the heart is primarily involved but the nervous system is not, the disease is called familial amyloidotic cardiomyopathy (FAC). A third major type of TTR amyloidosis is called leptomeningeal/CNS (Central Nervous System) amyloidosis.
Treatment of familial TTR amyloid disease has historically relied on liver transplantation as a crude form of gene therapy (Holmgren et al. Lancet 341: 1113-6). Inhibitory RNA therapies offer the prospect of a targeted and significantly less invasive alternative to such transplant therapy.
Double-stranded RNA (dsRNA) agents possessing strand lengths of 25 to 35 nucleotides have been described as effective inhibitors of target gene expression in mammalian cells (Rossi et al., U.S. Patent Application Nos. 2005/0244858 and US 2005/0277610). dsRNA agents of such length are believed to be processed by the Dicer enzyme of the RNA interference (RNAi) pathway, leading such agents to be termed “Dicer substrate siRNA” (“DsiRNA”) agents. Additional modified structures of DsiRNA agents were previously described (Rossi et al., U.S. Patent Application No. 2007/0265220). Effective extended forms of Dicer substrates have also recently been described (see, e.g., Brown, U.S. Pat. No. 8,349,809 and U.S. Pat. No. 8,513,207).
Provided herein are improved nucleic acid agents that target transthyretin. In particular, those targeting transthyretin have been specifically exemplified.